Microsporidia, a Highly Adaptive Organism and Its Host Expansion to Humans

Abstract

Emerging infectious disease has become the center of attention since the outbreak of COVID-19. For the coronavirus, bats are suspected to be the origin of the pandemic. Consequently, the spotlight has fallen on zoonotic diseases, and the focus now expands to organisms other than viruses. Microsporidia is a single-cell organism that can infect a wide range of hosts such as insects, mammals, and humans. Its pathogenicity differs among species, and host immunological status plays an important role in infectivity and disease severity. Disseminated disease from microsporidiosis can be fatal, especially among patients with a defective immune system. Recently, there were two Trachipleistophora hominis, a microsporidia species which can survive in insects, case reports in Thailand, one patient had disseminated microsporidiosis. This review gathered data of disseminated microsporidiosis and T. hominis infections in humans covering the biological and clinical aspects. There was a total of 22 cases of disseminated microsporidiosis reports worldwide. Ten microsporidia species were identified. Maximum likelihood tree results showed some possible correlations with zoonotic transmissions. For T. hominis, there are currently eight case reports in humans, seven of which had Human Immunodeficiency Virus (HIV) infection. It is observed that risks are higher for the immunocompromised to acquire such infections, however, future studies should look into the entire life cycle, to identify the route of transmission and establish preventive measures, especially among the high-risk groups.

Keywords: Trachipleistophora hominis; disseminated microsporidiosis; microsporidia; opportunistic infection; zoonosis.

Figure 1

Life cycle of T. hominis under laboratory conditions (Weidner et al., 1999). (1) Larvae were placed in water with T. hominis spores. (2) Spores observed in the gut (3). After several hours, the majority of spores germinated (4). After 1 week, the larvae matured into 3^rd instars and showed significant infection in their muscles. (Merogony and sporogony occurred during this stage) (5). Almost all microsporidia reached the spore stage before developing into pupae. (6) Spores were found in muscles, hemocoel, proboscis and, the cecal region. A low number of spores was detected in sugar water during feeding. The mosquito image here was taken from freepik.com.

Figure 2

Proposed T. hominis transmission in human beings (Heinz et al., 2012; Becnel and Andreadis, 2014; Watson et al., 2015). From the literature, the infected potential hosts can pass the spores with their secretion or feces. The dead bodies, once decay, can also release the resistant spores to the surroundings (1). The spores can persist outside for a long time because of the protective chitin shell (2). When a healthy host ingests the spores from the environment, they become infected. People can get the spores by inhalation (3), ingestion of contaminated water (4), and directly crushing the affected insect into the bite wound (5).

Figure 3

Maximum likelihood tree of microsporidia that cause disseminated disease in humans and sequences of the same species of other hosts were included. ML tree was constructed in Mega software version X using Kimura-2 parameter model. The sequences included were based on microsporidia species with reports of causing disseminated diseases in humans. Other nucleotide sequences detected in other organisms were included to find links of zoonosis. T. hominis species are closely related to A. algerae which is congruent with previous reports.

Figure 4

Hematoxylin & eosin stain of T. hominis in skeletal muscle (magnification X1000). Spores observed in circular spaces (parasitophorous vacuoles) in muscle fiber (black arrow). Spore with a characteristic belt-like stripe (red arrow). This image was created with BioRender (http://biorender.com/).